Differences in the rate of digestion of DNA rat liver nuclei of control, thyroidectomized and T3-treated thyroidectomized rats revealed that chromatin from thyroidectomized liver nuclei was less easily solubilized by nuclease than chromatin from normal liver nuclei. It is suggested that thyroid hormone causes changes in nucleoproteins which alter the structure of chromatin in such a way as to expose more DNA to nuclease attack and/or increase the dolubility of released nucleosomes. To study the mechanism of action of thyroid hormones, polyadenylated RNA must be prepared and specific antibody stimulated. Malic enzyme from fully stimulated rats was purified by affinity chromatography. This material was used to stimulate goat antibody. Total RNA, containing polyadenylated RNA, was prepared from the same group of rats as malic enzyme. This RNA translated proteins of different sizes and malic enzyme was immunologically precipitated using prepared antibody. Underivatized thyroid hormones were used for a covalent labeling of rat liver nuclear proteins. Results show the presence of a wide range of nuclear proteins which can be labeled with 2 nM of 125I-T4. The proteins which responded best were those having a molecular weight of 96,000, 56,000, 45,000 and 12-15,000.